In a conventional printer, sheets are passed from the printer's input tray to its platen using a plurality of rollers which pull consecutive sheets from a sheet media stack. The rollers rotate against the upper surface of the stack, frictionally directing advancement of the stack's top sheet downstream along a predetermined sheet media path. The sheet is advanced through an infeed zone and into a processing zone for printing, and then is expelled through an output zone and the cycle is begun anew.
Despite the apparent simplicity of this procedure, various problems have arisen due to the mechanism by which most printers direct sheet How. Such problems are particularly prevalent during sheet media infeed, an operation which involves the pick-up and separation of media sheets. These tasks generally are accomplished using systems which continuously urge the input tray toward the rollers, resulting in a constant engagement of the rollers by to-be-fed sheets. Although effective in accomplishing sheet media pick-up, such systems have been troubled by problems such as sheet scarring, component wear, and excessive power drain. In addition, known systems have made it difficult to add sheet media to the input tray, and have encountered difficulties in adequately addressing the problem of sheet media skew.
One solution to these problems has been to employ separate roller arrangements for the pick-up and post-pick-up advancement of sheets. In such systems, pick-up rollers frictionally advance a sheet to a main drive roller (or post-pick-up roller), and then disengage the sheet media stack until it is necessary to advance the next sheet. The main drive roller is thus able to advance the sheet through a processing cycle without the power drain which is inherent in sheet media pick-up. An illustrative system is set forth in U.S. Pat. No. 4,990,011, which names Underwood et al. as inventors, and which is commonly owned herewith. That patent describes a circular main drive roller and a somewhat unconventional D-shaped pick roller which is configured for rotational orientation either to engage or clear the sheet. During sheet media pick-up, the pick roller engages the sheet. During sheet processing, the pick-up roller remains spaced from (or clears) the sheet. The disclosure contained in that patent is incorporated herein by this reference.
Although the use of D-shaped pick rollers is effective in reducing sheet media drag, such an arrangement raises new issues related to requirements of roller timing and size. This is particularly true in view of sheet skew, a problem which presents itself due to less-than-perfect frictional advancement of a sheet. Paper skew generally is corrected by reverse drive of a sheet's leading edge by the main drive roller arrangement while the sheet's trailing edge is held in place. Such reverse drive results in a sheet bulge which effectively biases the sheet's leading edge so as to correct sheet skew.
It will thus be appreciated that, where D-shaped pick rollers are employed, the pick rollers must be positioned at a precisely determined distance from the main drive roller so as to ensure that the sheet will be taken into the main drive roller just prior to the pick roller reaching the orientation in which the roller clears the sheet stack. The pick roller will thus be able to hold the sheet's trailing edge in position while the main drive roller is reversed. A slight pick roller rotation will then place the pick roller in an orientation in which it clears the sheet stack. D-shaped roller arrangements have thus required complex timing mechanisms to ensure that the roller is in the sheet-clearing orientation during processing of a sheet.
The size of a D-shaped roller is similarly precisely determined, it being necessary to ensure that a picked sheet reach the main drive roller before the pick roller's rolling surface clears the sheet. If the pick roller were to progress to the sheet-clearing orientation prior to the sheet reaching the drive roller, an intermittent pause in sheet advancement would result. Such a pause would lead to an undesirable noise and would potentially leave the sheet susceptible to increased sheet skew. D-shaped pick rollers have therefore been relatively large in size so as to provide for adequate advancement of sheets.
It is therefore a general object of the invention to provide a simple, yet effective, system for advancing sheets. More specifically, it is an object of the this invention to provide a sheet advancement system which employs a roller arrangement which addresses the problems associated with conventional sheet media pick-up without requiring complex timing mechanisms or rollers which are unnecessarily large.